The invention relates to a steel material for hot work tools, i.e. tool for forming or working metals at comparatively high temperatures.
The term xe2x80x98hot work toolsxe2x80x99 is applied to a great number of different kinds of tools for the working or forming of metals at comparatively high temperatures, for example tools for die casting, such as dies, inserts and cores, inlet parts, nozzles, ejector elements, pistons, pressure chambers, etc.; tools for extrusion tooling, such as dies, die holders, liners, pressure pads and stems, spindles, etc.; tools for hot-pressing, such as tools for hot-pressing of aluminium, magnesium, copper, copper alloys and steel; moulds for plastics, such as moulds for injection moulding, compression moulding and extrusion; together with various other kinds of tools such as tools for hot shearing, shrink-rings/collars and wearing parts intended for use in work at high temperatures. There are a number of standard steel qualities used for these hot work tools, e.g. AISI Type H10-H19, and also several commercial special steels. Table 1 presents some of these standardised and/or commercial hot work steels.
In the first phase of the invention, the steels 1-15 in Table 1 were studied. This study indicated that none of the steels studied satisfied the demands that can be placed on tools for all the different areas of application mentioned above. Consequently, subsequent work concentrated on the development of an alloy primarily intended for die casting of light metals, an area of application where there is a special need of a new steel material with a combination of properties that is better than that currently available using known steels. The objective of the steel material in accordance with the invention is to offer optimal properties in terms of good hardenability and microstructure in order to provide high levels of toughness and ductility also in heavy gauges. At the same time there must be no deterioration of tempering resistance and high temperature strength.
More particularly, a purpose of the invention is to offer a hot work steel with a chemical composition that is such that the steel can satisfy the following demands:
it must have good hot workability in order to thereby get a high yield on manufacture,
it should be capable of manufacture in very heavy gauges, which means thicker than e.g. 760xc3x97410 mm or thicker than Ø 550 mm,
it should have very low content of impurities,
it should not contain any primary carbides,
it should have good hot treatment properties, meaning inter alia that it should be capable of being tempered at a moderately high austenitizing temperature,
it should have very good hardenability, i.e. it should be capable of being through-hardened even in the above-mentioned very heavy gauges,
it should be form-stable during heat treatment,
it should have good tempering resistance,
it should have good high-temperature strength,
it should have very good toughness and very good ductility properties in the dimension ranges in question,
it should have good thermal conductivity,
it should not have an unacceptably large coefficient of heat expansion,
it should have good coating properties with PVD/CVD/nitriding,
it should have good spark erosion properties, good cutting and welding properties, and
it should have a favourable manufacturing cost
The above-mentioned conditions can be satisfied by the invented steel material for the following reasons: firstly, by the steel alloy having such a basic composition that the material can be processed in order to yield an adequate microstructure with very even distribution of carbides in a ferritic matrix, suitable for further heat treatment of the finished tool; secondly, by the steel material with the said basic composition also having the prescribed low contents of silicon, which is to be regarded as an impurity in the steel of the invention, and also very low contents of the non-metallic impurities nitrogen, oxygen, phosphor and sulphur. Indeed it has long been known that non-metallic impurities, such as sulphur, phosphor, oxygen and nitrogen, involve certain negative effects for many steels, especially regarding the toughness of the steel. This also applies concerning the knowledge that some metals in trace element levels may have negative effects for many steels, such as reduced toughness. For instance, this applies in relation to titanium, zirconium and niobium at small levels. Nonetheless, it has not been possible in the case of most steels, including hot work steel, to improve toughness significantly solely by reduction of contents of impurities of this nature in steel. The study conducted of existing steel alloys has also demonstrated that good toughness cannot be attained solely by optimising the basic composition of the steel alloy. It was only possible to attain the said conditions by a combination of an optimal basic composition and low or very low contents of the said non-metallic impurities, and also preferably a very low content of titanium, zirconium and niobium.
In order to satisfy the above-mentioned conditions the invented steel material has an alloy composition that by weight-percentage essentially consists of:
0.3-0.4 C, preferably 0.33-0.37 C, typically 0.35 C
0.2-0.8 Mn, preferably 0.40-0.60 Mn, typically 0.50 Mn
4-6 Cr, preferably 4.5-5.5 Cr, suitably 4.85-5.15 Cr, typically 5.0 Cr
1.8-3 Mo, preferably max. 2.5 Mo, suitably 2.2-2.4 Mo, typically 2.3 Mo
0.4-0.6 V, preferably 0.5-0.6 V, suitably 0.55 V, balance iron and unavoidable metallic and non-metallic impurities, in connection said non-metallic impurities comprising silicon, nitrogen, oxygen, phosphor and sulphur, which may be included up to the following maximum contents:
max. 0.25 Si, preferably max. 0.20 Si, suitably max. 0.15 Si
max. 0.010 N, preferably max. 0.008 N
max. 10 ppm O, preferably max. 8 ppm O
max. 0.010 P, preferably max. 0.008 P, and
max. 0.010 S, preferably max. 0.0010, suitably max. 0.0005 S
It is preferable that titanium, zirconium and niobium occur in the following maximum contents by weight-%
max. 0.05 Ti, preferably max. 0.01, suitably max. 0.008,
and most preferably max. 0.005,
max. 0.1, preferably max. 0.02, suitably max. 0.010,
and most preferably 0.005 Zr,
max. 0.1, preferably max. 0.02, suitably max. 0.010,
and most preferably max. 0.005 Nb.
As regards the choice of individual desirable alloy elements, it can be briefly stated that the contents of carbon, chromium, molybdenum and vanadium have been chosen so that the steel should have a ferritic matrix in the delivery condition of the material, a martensitic matrix with adequate hardness after hardening and tempering, absence of primary carbides but the existence of secondary precipitated carbides of MC and M23C6 type of sub-microscopic size in the hardened and tempered material, while at the same time the basic composition of the steel shall provide potential in order to also attain the desired toughness.
The minimum content of chromium shall be 4%, preferably 4.5% and suitably at least 4.85% in order that the steel should have adequate hardenability but may not be included at contents exceeding 6%, preferably max. 5.5% and suitably max. 5.15% in order that the steel should not result in carbide content of type M23C6 and M7C3 to an undesirable extent after tempering. The nominal chromium content is 5.0%.
Tungsten adversely affects thermal conductivity and hardenability in relation to molybdenum and is therefore not a desirable element in the steel but may be permitted in contents up to 0.5%, preferably max. 0.2%. However, the steel should suitably not contain any intentionally added tungsten, i.e. the most desirable form of the steel only contains tungsten at impurity levels.
Molybdenum should be included at a minimum content of 1.8%, preferably at least 2.2% in order to provide adequate hardenability and tempering resistance together with the desirable high temperature strength properties. Greater contents of molybdenum than 3% carry a risk of grain boundary carbides and primary carbides, which reduce toughness and ductility. Molybdenum should therefore not be included at higher contents than 3.0%, preferably max. 2.5%, suitably max. 2.4%. If the steel contains a certain content of tungsten in accordance with the above, tungsten partly substitutes molybdenum in accordance with the rule xe2x80x9ctwo parts tungsten corresponds to one part molybdenumxe2x80x9d.
The steel shall contain a content of at least 0.4% vanadium to provide an adequate tempering resistance and desired high temperature strength properties. Furthermore, the vanadium content should be at least the stated content to prevent grain coarsening when heat treating the steel. The upper limit for vanadium of 0.6% is set to reduce the risk of formation of primary and grain boundary carbides and/or carbonitrides, which would reduce the ductility and toughness of the steel. The steel should preferably contain 0.5-0.6 V, suitably 0.55 V.
The steel should contain manganese in the stated levels, primarily to increase the hardenability to some degree.
In order to utilise the potential good toughness that a steel material with the said contents of carbon, manganese, chromium, molybdenum and vanadium can provide, the contents on the said non-metallic impurities should at the same time be held at the said low or very low levels. The following may be said regarding the significance of these elements of impurity.
Silicon can be found as a residual product in the steel from its de-oxidation and may be included at a highest level of 0.25%, preferably max. 0.20% and suitably max. 0.15% in order that the carbon activity should be kept low and consequently even the content of primary carbides that can be precipitated during the solidification process, and, at a later phase, also the grain boundary carbides, which improves toughness.
Nitrogen is an element that tends to stabilise primary carbide formation. Primary carbonitrides, in particular carbonitrides in which, besides vanadium, titanium, zirconium and niobium may be included, are more difficult to dissolve than pure carbides. These carbides, if they are present in the finished tool, may have a major negative effect on the impact toughness of the material. With very low contents of nitrogen, these carbides are dissolved more readily on the austenitizing of the steel in conjunction with heat treatment, following which the said small secondary carbides, primarily MC and M23C6 type of sub-microscopic size, i.e. less than 100 nm, normally 2-100 nm, are precipitated, which is advantageous. The steel material according to the invention should therefore contain max. 0.010% N, preferably max 0.008% N.
Oxygen in the steel forms oxides, which can initiate fractures as a result of thermal fatigue. This negative effect on ductility is counteracted by a very low content of oxygen, max. 10 ppm O, preferably max. 8 ppm O.
Phosphor segregates in phase boundary surfaces and grain boundaries of all kinds and reduces cohesion strength and consequently toughness. Phosphor content should therefore not exceed 0.010%, preferably max. 0.008%.
Sulphur which by combining with manganese forms manganese sulphides, has a negative effect on ductility but also on toughness because it influences transverse properties negatively. Sulphur may therefore exist in an amount of max 0.010%, preferably max 0.0010%, suitably max. 0.0008%.
Titanium, zirconium and niobium content ought not to exceed levels in the steel higher than the maximum contents mentioned above, i.e. max. 0.05% Ti, preferably max. 0.01, suitably max. 0.008 and most preferably max. 0.005 Ti, max. 0.1, preferably max. 0.02, suitably max. 0.010 and most suitably 0.005 Zr and max. 0.1, preferably max. 0.02, suitably max. 0.010,and most preferably max. 0.005 Nb, in order to avoid the formation of nitrides and carbonitrides primarily.
In its delivery condition, the steel material according to the invention has a ferritic matrix with evenly distributed carbides, that are dissolved on the heat treatment of the steel in conjunction with hardening. On this heat treatment the steel is austenitized at a temperature between 1000 and 1080xc2x0 C., suitably at a temperature of 1020-1030xc2x0 C. The material is thereafter cooled to room temperature and tempered one or several times, preferably 2xc3x972 h, at 550-650xc2x0 C., preferably at approx. 600xc2x0 C.
Further characteristics and aspects of the invention will be apparent from the following description of experiments conducted and from the appending patent claims.